Continuation Analysis

Because dynamic relaxation is iterative, it is sometimes desirable to start the iterations from a particular point in the analysis other than the initial position.

The initial position is the undeformed structure at rest with no forces other than prestress in the elements. As the iterations progress, the elements deflect and their internal forces increase until the deflected shape and internal forces are in equilibrium with the applied loads. If further loading is then to be applied, or damping or convergence parameters adjusted, it is quicker to start iterating again from this deflected, loaded position. This is termed continuation analysis.

A particular case of continuation analysis is form finding. After form-finding analysis, options will be given to update the nodal coordinates and save the current member forces. In this case the nodal coordinates will be overwritten with the coordinates of the deflected shape, the equilibrium element forces will be saved as prestressed forces equivalent distortion loads in an extra load case.

Form finding can be used to carry out continuation analysis. For example, any model, (not only zero stiffness form-finding models), can be analysed with the form-finding option, ignoring the form finding properties. This will perform the same analysis as single increment analysis. Opting to modify the node coordinates and save current member forces at the end of the form-finding analysis will guarantee that the current deformed state of the structure has been kept. If the resulting model is re-analysed with the load case combination used in previous form-finding analysis plus the extra load case just created, the structure will not deform any more since it is already in the equilibrium state.

The usefulness of this option is that it allows the structure to be modified between the first step form-finding analysis and the second stage normal analysis. For example, extra loading, elements or constraints can be added to the structure after form-finding analysis, then the structure can be analysed again with the extra load case being included in the load combination. This process could be used to model the construction stage of a structure.